The most common system used for space cooling centers in the heat pump, which is based essentially on a refrigeration cycle involving the absorption of heat by the evaporation of a compressed refrigerant, followed by re-compressing the refrigerant and condensing it by transferring heat to a medium that may be convenient for disposal or heat utilization. The absorption phase can be adapted to cool the air directly with a heat exchanger, or cool some other medium to which the air is exposed. The condensation phase usually utilizes a heat exchanger that is cooled by a reference medium, such as outside air or ground water.
The efficiency of such a system is determined by comparing the space heat withdrawn, and the operating watts, giving the "energy efficiency ratio", or EER, commonly less than 14% for ground-water reference systems. A system referenced to outdoor air can be expected to operate under an EER of around 10%. A system in which space heat can be transferred directly to ground water (without a refrigeration cycle) can conceivably operate under an EER of 50 to 100, which is a tempting alternative. Here, the expended energy is primarily in the operation of circulation pumps.
Space-cooling systems should maintain humidity in the range of 40%-60%. Air out of the system and into the space should be at about 60.degree. to be raised to around 78.degree. when mixed with room air. Ground water in the northern Midwest can be expected to be at around 54.degree., and it is difficult to bring air down to 60.degree. using direct transfer of heat to ground water exclusively. The temperature differential is too small for good heat transfer. These are the conditions providing the stimulus for the present invention.